1. Field of the Invention
The present invention relates to a semiconductor laser device.
2. Description of the Related Art
Patent Document 1 (Japanese Unexamined Patent Application Publication No. 7-249829) describes a distributed feedback laser (DFB laser) formed on a semiconductor substrate. This DFB laser includes a p-type InP cladding layer, an optical guide layer, an active layer having a strained quantum well structure, and an n-type InP cladding layer. These layers are stacked on a p-type InP substrate in this order. A diffraction grating is formed at the interface between the p-type InP cladding layer and the optical guide layer. The p-type InP cladding layer and the n-type InP cladding layer have smaller refractive indices than the optical guide layer and the active layer. Specifically, the DFB laser has a single optical-waveguide structure constituted by the p-type InP cladding layer, the optical guide layer, the active layer, and the n-type InP cladding layer. In the optical-waveguide structure, the optical guide layer and the active layer are sandwiched between the p-type InP cladding layer and the n-type InP cladding layer. Therefore, the optical guide layer and the active layer are core region of the optical waveguide. Light generated in the active layer is confined within the optical guide layer and the active layer.